This invention relates generally to medical imaging systems, and more particularly, to emission mammography systems.
Imaging devices, such as gamma cameras or positron emission tomography scanners, are used in the medical field to detect radioactive emission events emanating from an object and to detect transmission or gamma rays attenuated by the object. An output, typically in the form of an image that graphically illustrates the distribution of the emissions within the object and/or the distribution of attenuation of the object is formed from these detections. The detector of an imaging device detects the number of emissions, for example, gamma rays in the range of about seventy keV to about six hundred keV, and may detect gamma rays that have passed through the object.
For example, single photon emitting-radionuclides, such as 99mTc, emit one gamma ray in a radioactive transition. These gamma rays are detected (typically by a scintillation detector, such as thallium-activated sodium iodide, NaI(Tl)) and are used to generate an image of the spatial distribution of the radionuclide. In addition, positron emission imaging generates images that represent the distribution of positron-emitting nuclides within the body of a patient. When a positron interacts with an electron by annihilation, the entire mass of a positron-electron pair is converted into two 511-keV photons. The photons are emitted in opposite directions along a line of response. The annihilation photons are detected by detectors that are placed on both sides of the line of response. When these photons arrive and are detected by the detector elements within a preset time interval, this is referred to as a true coincident event. An image is then generated based on the acquired image data that includes the annihilation photon detection information.
Different systems and methods for performing mammography imaging are known. For example, it is known to use x-ray systems for film screen mammography imaging. This type of mammography uses transmitted x-rays to produce an image of the breast. This type of mammography results in an image that represents the spatial distribution of the x-ray differential attenuation throughout the breast. This type of mammography imaging may not perform satisfactory imaging in women with dense breasts and further may not be able to provide sufficient differentiation to distinguish between benign and malignant lesions.
It is also known to perform Positron Emission Mammography (PEM) wherein radioactively tagged molecules (e.g., 18F-FDG) are injected into a patient to be imaged to mark or make more visible tissue that is more likely to be cancerous. Tumor cells show up as hot spots in the PEM images. It is also known to use single photon-emitting isotopes (such as 99mTc) to perform scintimammography. Gamma cameras are thereby used to provide scintigraphic imaging wherein a radioactive tracer is employed and injected into a vein to identify abnormal cells based on the difference in physiological characteristics between cancer cells and non-cancer cells. The gamma camera is used to localize the radioactive tracer in the breast. Essentially, a gamma camera is placed at one or both sides of the breast to provide imaging of radiotracer distribution in the breast. Scintimammographic imaging systems also may use different types of detectors, such as, for example, cadmium zinc telluride (CZT) detectors.
Thus, emission mammography is typically performed using a conventional gamma camera, often referred to as scintimammography, with the patient either prone and the breast dependent or in lateral recumbence with the homolateral arm raised. Additionally, a gamma camera with a single detector can be used to emulate the positioning of a conventional x-ray mammography unit for nuclear medicine based studies. However, all of these known systems provide imaging of the breast in only a single view. Thus, quantitative measurement of in vivo activity contained within, for example, a tumor, cannot be reasonably or accurately obtained from single planar views because the thickness of both the overlying anatomy and the tumor are unknown.